1. Field of the Invention
The present invention relates generally to measurement techniques for performing adsorption and desorption gas analyses on materials, and more particularly to an improved technique and apparatus for simultaneously and independently performing gas adsorption and desorption measurements on a plurality of different samples, such as powder or pellet samples, for example.
Dozens of patents have been issued for method and apparatus for measuring gas adsorption of samples The prior art most pertinent to the present invention involves methods and apparatus which use absolute pressure transducers in the measurement of adsorption on a plurality of samples simultaneously, and methods and apparatus which use differential pressure transducers for purposes different than those of the present invention.
A great variety of applications in modern technology require accurate information concerning the microstructure of materials such as powders which are widely used, for example, as catalysts or in the production of paint, cement, and the like. This information includes the porosity and surface areas of the powder material and the distribution of pore volume in the various sized pores.
One conventional manner of performing such measurements, which is disclosed, for example, in U.S. Pat. No. 3,555,912 to Lowell (Lowell '912), involves the preparation of adsorption and desorption isotherms of the material in which volume is plotted against the equilibrium relative pressure of the gas (typically nitrogen) adsorbed or desorbed on the surface of the material. To prepare such isotherms, the quantity of the gas adsorbed onto the solid surface or desorbed from the solid surface is measured at various specified pressures at a specific temperature. The conventional gas adsorption and desorption processes are, however, relatively slow, often requiring hours or sometimes even days to acquire the necessary data to accurately characterize the porosity and surface area of the material.
Many types of vacuum-volumetric apparatus and systems have been developed for performing adsorption and desorption measurements on samples. Many of these adsorption and desorption measurement systems have certain essential features. These include a non-adsorbable gas (e.g. helium) for calibrating the volume of the cell containing the sample, (except for the so called NOVA system manufactured by Quantachrome Corporation of Boynton Beach, Fla.), U.S. Pat. No. 5,360,743 to Lowell, which avoids the need for such calibration and another technique to be described later) and an adsorbate gas (e.g. nitrogen) for performing the adsorption or desorption analysis. The non-adsorbable gas is critical in these systems for calibrating the volume of the sample cell with the sample material present.
The prior art has speeded the production of analytical data to some extent. U.S. Pat. No. 4,566,326 to Lowell (Lowell '326) provides an analyzer unit capable of concurrently and independently obtaining data for a plurality of powder samples in a single measuring instrument.
It is the main object of the aforesaid patented invention to provide an analyzer unit which is capable of acquiring data on several samples substantially simultaneously and in which the data derived for a plurality of samples can be stored and presented in any of a number of different forms for each of the samples. To these ends, the automatic measurement units of the invention described and included in the aforesaid Lowell '326 patent includes a common manifold in respective fluid communication through a plurality of isolation valves with a corresponding plurality of sample cells, and absolute pressure transducers are provided to measure the manifold pressure, and a plurality of absolute pressure transducers are respectively coupled to the sample cells (a separate absolute pressure transducer for each sample cell) to provide measurement of the respective pressures in the sample cells.
The Lowell '326 invention operates automatically under the control of a microprocessor, which receives and processes information from the several pressure transducers and controls the operation of the isolation valves. This microprocessor measures current cell pressure, manifold temperature and manifold pressure. When the manifold and cell are connected through an opened isolation valve, a pressure in the combined volume results. This latter pressure is used to calculate a specified relative pressure. The manifold is then pressurized to the calculated value with nitrogen gas, and the cell isolation valve is opened to allow a volume of nitrogen to be admitted (or removed) into (or from) one of the sample cells which contain the solid adsorbent sample to be analyzed. The isolation valve is then closed. As adsorption (or desorption) of the gas occurs in the sample, the pressure in the sample cell as measured periodically by the cell pressure transducer varies until an equilibrium pressure or relative pressure is attained. Relative pressure is defined as the ratio of the equilibrium pressure (P) to the saturated vapor pressure (P.sub.o), expressed as P/P.sub.o. The quantity of the gas adsorbed (or desorbed) at the equilibrium pressure is the difference between the amount of gas admitted (or removed) and the amount required to fill the space around the adsorbent (void space) which was previously determined with non-adsorbable helium gas.
This process is repeated for different volumes and relative pressures until a preselected number of data points have been produced for the sample. At one of several stages in the analysis sequence of a given sample cell at which an interval or delay is required, the processing of analysis is shifted for that period to another of the sample cells according to a priority programmed into the microprocessor. This process of shifting the operations between the sample cells on a priority basis continues in a sequence determined to achieve optimum analysis, until the required pressure-volume data points for all of the samples under analysis have been obtained.
Other pertinent prior publications relevant to this invention in addition to the aforesaid Lowell '326 patent include a book entitled "Adsorption, Surface Area and Porosity" by S. J. Gregg and K. S. W. Sing, published in 1967 by Academic Press, London and New York (particularly pages 324 to 326); a "Strohlein AREA-Meter" brochure and U.S. Pat. No. 5,133,219 to Camp (Camp '219).
As will be explained later, these latter publications incorporate a differential pressure transducer between a sample cell and a comparison cell. In the Strohlein AREA-Meter and Camp '219, the comparison cell is used as a reference only and is not used with a sample in the cell. The sole purpose of the differential gauge and the reference cell is to determine the amount of gas not adsorbed on the sample surface in the sample cell. These publications differ from the present invention in that results obtained in the prior art are not acquired as rapidly as are obtainable with the present invention. It is believed that the prior art fails to obtain the rapid obtention of data possible with the present invention because the prior art measures only one sample cell at a time.
U.S. Pat. No. 5,360,743 to Lowell (Lowell '743) provides an improved apparatus and method for measuring the void volume, and the adsorption of the sample cell walls, and correcting for non-ideal gas behavior by using only the adsorbate gas. This Lowell invention effectively overcomes the aforementioned difficulties and longstanding problems inherent in surface area and pore volume measurements. These problems have been solved in a simple, convenient, and highly effective way by which to increase the accuracy of the measurement and to decrease the time consumed in performing the measurements.
Despite the improvements developed in the technology as evidenced by the patents described hereinbefore, there still remains room for further improvements that enable the art to obtain the requisite data even more rapidly using modified apparatus that is less costly in that the present apparatus eliminates an expensive component previously required in the prior art as will be described hereafter.